just-mcp

👋 A way to let LLMs speak Just

A production-ready MCP server that provides seamless integration with Just command runner, enabling AI assistants to discover, execute, and introspect Justfile recipes through the standardized MCP protocol.

🎯 Why Just + MCP = Better Agent Execution

Context-Saving Abstraction

If it isn't immediately obvious, the benefit of having LLMs use Just vs. bash is that running Just commands (via MCP) provides a context-saving abstraction where they don't need to waste context opening/reading bash files, Python scripts, or other build artifacts. The LLM via MCP simply gets the command, parameters, and hints - it's in their memory as "these are commands available to you."

Eliminates the Justfile Learning Curve

No more watching LLMs execute just -l to get command lists, inevitably start reading the justfile, then try to write justfile syntax (like it's a Makefile), corrupt the justfile, and create a bad experience. Just's evolving syntax simply doesn't have a large enough corpus in frontier models today - we need more popular repos with justfiles in the training dataset.

Safer Than Raw Bash Access

Just-mcp is fundamentally safer than bash. If you read HackerNews, there's a story at least once daily about operators whose LLMs start forgetting, hallucinating, and eventually breaking down - deleting files and doing nasty unwanted things. Giving LLMs unsupervised, unrestricted bash access without carefully monitoring context consumption is a recipe for disaster.

Using Justfile fixes that. Even if the LLM modifies its own justfile, the next context is memoized by the justfile (hopefully in an idempotent git repo). This abstraction shields the llm from the command line complexity where hallucinations or attention tracking the current working directory cause it to go over the rails and off the cliff.

Powerful Agent Execution Tool

Just-mcp is perfect for anybody doing agent execution:

• Ultra-low overhead - probably better than every other tool
• Human-friendly - justfiles are easy for humans and low overhead for LLMs
• Quick and dirty - while some prefer full Python FastAPI servers, just-mcp is just easy-as
• sm0l model friendly - works great with self-hostable GPU/CPU open source models with 8k-32k context limits

Built-in Safety Patterns

Just has useful patterns for introducing:

• Transparent logging without distracting the agent
• Secondary model inspection - use sm0l models to scan commands asking "is this harmful?" before execution
• Python decorator-like patterns for command validation
• Idempotent execution backed by git repos

b00t

b00t mcp create just-mcp -- bash just-mcp --stdio "${REPO_ROOT}"
b00t mcp export just-mcp

🚀 Current Status: 67% Complete (8/12 core tasks)

✅ Implemented Features

• 🏗️ Complete MCP Server - Full rmcp 0.3.0 integration with MCP 2024-11-05 protocol
• 📋 Recipe Discovery - Parse and list all available Justfile recipes
• ⚡ Recipe Execution - Execute recipes with parameters and capture structured output
• 🔍 Recipe Introspection - Get detailed recipe information, parameters, and documentation
• ✅ Justfile Validation - Syntax and semantic validation with error reporting
• 🌍 Environment Management - Comprehensive .env file support and variable expansion
• 🧪 Full Test Coverage - 33 passing tests across integration and unit test suites

🎯 MCP Tools Available

1. list_recipes - List all available recipes in the justfile
2. run_recipe - Execute a specific recipe with optional arguments
3. get_recipe_info - Get detailed information about a specific recipe
4. validate_justfile - Validate the justfile for syntax and semantic errors

🏃 Quick Start

Installation

Choose your preferred installation method:

npm (JavaScript/TypeScript)

# Install globally
npm install -g just-mcp

# Or use with npx (no installation required)
npx just-mcp --stdio

pip (Python)

# Install with pip
pip install just-mcp

# Or use with uvx (recommended)
uvx just-mcp --stdio

Cargo (Rust)

# Install from crates.io
cargo install just-mcp

# Or build from source
git clone https://github.com/promptexecution/just-mcp
cd just-mcp
cargo build --release

pkgx (pkgxdev)

pkgx just-mcp --stdio

pkgx downloads the platform-specific tarball that GitHub releases expose (just-mcp-*-*.tar.gz), extracts the executable into ${PKGX_DIR:-$HOME/.pkgx}/bin, and runs the CLI with the arguments you pass. Add that bin directory to your shell’s PATH if you need just-mcp available long-term. The packaging manifest lives in pkgx/projects/github.com/promptexecution/just-mcp/package.yml and mirrors the pkgxdev/pantry entry.

Using Docker

# Pull the latest image from GitHub Container Registry
docker pull ghcr.io/promptexecution/just-mcp:latest

# Run with Docker
docker run --rm -v $(pwd):/workspace ghcr.io/promptexecution/just-mcp:latest --stdio

# Build locally
docker build -t just-mcp:local .
docker run --rm -v $(pwd):/workspace just-mcp:local --stdio

Available Docker image tags:

• latest - Latest stable release
• X.Y.Z - Specific version (e.g., 0.1.0)
• X.Y - Latest patch version (e.g., 0.1)
• X - Latest minor version (e.g., 0)

Claude Desktop Integration

Using npm/npx

Add to your Claude Desktop MCP configuration:

Using Binary

{
  "mcpServers": {
    "just-mcp": {
      "command": "npx",
      "args": ["-y", "just-mcp", "--stdio"]
    }
  }
}

Using pip/uvx

{
  "mcpServers": {
    "just-mcp": {
      "command": "uvx",
      "args": ["just-mcp", "--stdio"]
    }
  }
}

Using cargo or manual install

{
  "mcpServers": {
    "just-mcp": {
      "command": "/path/to/just-mcp",
      "args": ["--stdio"]
    }
  }
}

Using Docker

{
  "mcpServers": {
    "just-mcp": {
      "command": "docker",
      "args": [
        "run",
        "--rm",
        "-i",
        "-v",
        "${workspaceFolder}:/workspace",
        "ghcr.io/promptexecution/just-mcp:latest",
        "--stdio"
      ]
    }
  }
}

Usage Examples

# Run as MCP server
just-mcp --stdio

# Run in specific directory  
just-mcp --directory /path/to/project --stdio

# Using Docker
docker run --rm -v $(pwd):/workspace ghcr.io/promptexecution/just-mcp:latest --stdio

🧪 Testing

Comprehensive Test Suite

# Run all tests (33 tests)
cargo test

# Run specific test suites
cargo test --test basic_mcp_test      # Protocol compliance testing
cargo test --test mcp_integration_working  # SDK integration testing

Test Architecture

• basic_mcp_test.rs - Direct protocol compliance testing using raw JSON-RPC
• mcp_integration_working.rs - Type-safe SDK integration testing with rmcp client
• Unit tests - 25+ tests covering parser, executor, validator, and environment modules

📚 Architecture

Project Structure

just-mcp/
├── src/main.rs              # CLI binary
├── just-mcp-lib/           # Core library
│   ├── parser.rs           # Justfile parsing
│   ├── executor.rs         # Recipe execution  
│   ├── validator.rs        # Validation logic
│   ├── environment.rs      # Environment management
│   └── mcp_server.rs       # MCP protocol implementation
├── tests/                  # Integration tests
└── justfile               # Demo recipes

Tech Stack

• Rust 1.82+ with async/await support
• rmcp 0.3.0 - Official MCP SDK for Rust
• serde/serde_json - JSON serialization
• snafu - Structured error handling
• tokio - Async runtime

🔄 Development Roadmap

🎯 Next Priority Tasks (Remaining 33%)

1. LSP-Style Completion System - Intelligent autocompletion for recipes and parameters
2. Enhanced Diagnostics - Advanced syntax error reporting and suggestions
3. Virtual File System - Support for stdin, remote sources, and in-memory buffers
4. Release Preparation - Documentation, CI/CD, and crate publication

🚀 Future Enhancements

• Plugin system for custom recipe types
• Integration with other build tools
• Performance optimizations for large justfiles
• Advanced dependency visualization

📖 Usage Patterns

Recipe Execution

// List available recipes
await client.callTool("list_recipes", {});

// Execute recipe with parameters  
await client.callTool("run_recipe", {
  "recipe_name": "build",
  "args": "[\"--release\"]"
});

// Get recipe information
await client.callTool("get_recipe_info", {
  "recipe_name": "test"
});

Validation

// Validate justfile
await client.callTool("validate_justfile", {
  "justfile_path": "./custom.justfile"  
});

🤝 Contributing